Approximately 15% of women sustain pelvic muscle injuries during vaginal birth, demonstrable through magnetic resonance (MR) imaging. Our geometric model suggests the pelvic floor muscles stretch up to 3.3 times their original length during the second stage of birth. Based on MR imaging, histological, studies, and biomechanical modeling, when injury occurs, we hypothesize an initial rupture of the pubic attachment ofthe pubovisceral muscle (Type 1 injury), in some women followed by an avulsion of the pubic attachment of the arcus tendineus levator ani (Type 2 injury). We seek funds to characterize the variation of tissue in these injury zones and document their failure loads. AIM 1 will characterize the morphology and histology of pubovisceral and arcus tendineus tissues, bi-axial material properties where appropriate, and uniaxial failure properties in 30 cadavers. In 20 volunteers, we will use ultrasound elastography to characterize the effect of pregnancy on perineal body elasticity. AIM 2 will use Aim 1 data to refine a subject-specific, 3-D finite element viscohyperelastic biomechanical model of the second stage of labor from Station +2 on. We will test the above hypotheses with regard to the sequence of Type 1/Type 2 injuries, and run parametric studies to determine the biomechanical factors that increase or decrease the risk of Type 1 and Type 2 injuries. We will validate the model with stereophotogrammetric measurements of perineal descent in 10 laboring women. Since maternal exhaustion is a major risk for instrumented delivery and levator muscle injuries, in AIM 3 we will use a repeated measures design to quantify the outcomes in 40 healthy non-pregnant volunteers between 21 and 30 years of age to study the energetic cost of Valsalva pushing with and without (a) maximal arm and (b) arm and thigh muscle isometric contractions. These observations will yield insights into the factors associated with the greatest risk for injury, and should lead to better methods of preventing these injuries.